1. Field of the Invention
This invention relates to a radiation image detector, which is capable of generating electric charges when radiation carrying image information of an object is irradiated to the radiation image detector, and which is capable of recording a radiation image of the object through accumulation of the electric charges.
2. Description of the Related Art
Various radiation image detectors, which are capable of recording radiation images of objects when the radiation carrying image information of the objects is irradiated to the radiation image detectors, have heretofore been proposed and used in practice in medical fields, and the like.
Examples of the aforesaid radiation image detectors include the radiation image detectors utilizing amorphous selenium, which is capable of generating the electric charges when the radiation is irradiated to amorphous selenium. As the radiation image detectors utilizing amorphous selenium, there have been proposed the radiation image detectors employed for an optical read-out technique and the radiation image detectors employed for an electrical read-out technique.
FIG. 13 is an explanatory view showing positive hole injection in a conventional radiation image detector. As one of the radiation image detectors employed for the optical read-out technique, for example, as illustrated in FIG. 13, there has been proposed a radiation image detector comprising: (i) a first electrode layer 101, which has transmissivity to radiation carrying radiation image information, (ii) a recording photo-conductor layer 102, which generates electric charges when it is exposed to the radiation having passed through the first electrode layer 101, (iii) a charge transporting layer 103, which acts as an electrical insulator with respect to the electric charges having a certain polarity among the electric charges having been generated in the recording photo-conductor layer 102, and which acts as an electrical conductor with respect to the electric charges having an opposite polarity, (iv) a reading photo-conductor layer 104, which generates electric charges when it is exposed to reading light, and (v) a second electrode layer, which is constituted of transparent linear electrodes 106, 106, . . . having the transmissivity to the reading light and light blocking linear electrodes 107, 107, . . . for blocking the reading light, wherein the first electrode layer 101, the recording photo-conductor layer 102, the charge transporting layer 103, the reading photo-conductor layer 104, and the second electrode layer are overlaid in this order.
In cases where a radiation image is to be recorded on the radiation image detector employed for the optical read-out technique as described above, firstly, a negative voltage is applied from a high voltage electric power source to the first electrode layer 101 of the radiation image detector. Also, in this state, radiation carrying radiation image information of an object is irradiated from the side of the first electrode layer 101 of the radiation image detector.
The radiation, which has thus been irradiated to the radiation image detector, passes through the first electrode layer 101 and impinges upon the recording photo-conductor layer 102. As a result, pairs of positive and negative charges are generated in the recording photo-conductor layer 102 by the irradiation of the radiation. Of the pairs of positive and negative charges having been generated in the recording photo-conductor layer 102, the positive charges combine with the negative charges occurring in the first electrode layer 101 and become extinct. Of the pairs of positive and negative charges having been generated in the recording photo-conductor layer 102, the negative charges are accumulated as latent image charges at a charge accumulating section 105, which is formed at an interface between the recording photo-conductor layer 102 and the charge transporting layer 103. The radiation image is thus recorded as illustrated in FIG. 13.
In cases where the radiation image having thus been recorded is to be read out from the radiation image detector, the first electrode layer 101 is grounded. In this state, the reading light is irradiated from the side of the second electrode layer to the radiation image detector. The reading light, which has been irradiated from the side of the second electrode layer, passes through the transparent linear electrode 106 of the second electrode layer and impinges upon the reading photo-conductor layer 104. As a result, pairs of positive and negative charges are generated in the reading photo-conductor layer 104 by the irradiation of the reading light. Of the pairs of positive and negative charges having been generated in the reading photo-conductor layer 104, the positive charges combine with the latent image charges having been accumulated at the charge accumulating section 105. Also, of the pairs of positive and negative charges having been generated in the reading photo-conductor layer 104, the negative charges combine with the positive charges occurring in the transparent linear electrode 106 and the light blocking linear electrode 107. An electric current flowing at the time, at which the negative charges in the reading photo-conductor layer 104 and the positive charges of the transparent linear electrode 106 and the light blocking linear electrode 107 thus undergo the combination, is detected by a charge amplifier, which is connected to the light blocking linear electrode 107. In this manner, an image signal representing the radiation image having been recorded is read out from the radiation image detector.
However, as illustrated in FIG. 13, in cases where the first electrode layer 101 is grounded for the readout of the image signal after the radiation image has been recorded on the radiation image detector in the manner described above, positive holes are injected from the first electrode layer 101 into the recording photo-conductor layer 102 due to the effects of the electrons having been accumulated at the charge accumulating section 105. Due to the injection of the positive hole, noise is mixed in the image signal having been read out, and the image quality of the radiation image having been read out becomes bad.
Also, as one of the radiation image detectors employed for the electrical read-out technique, for example, there has been proposed a radiation image detector comprising: (i) a top electrode, to which a voltage is to be applied, (ii) a semiconductor layer, which is capable of generating electric charges when radiation is irradiated to the semiconductor layer, and (iii) an active matrix substrate, the top electrode, the semiconductor layer, and the active matrix substrate being overlaid one upon another, the active matrix substrate being constituted of a plurality of pixels arrayed in a two-dimensional pattern, each of the pixels being provided with (a) a collecting electrode for collecting electric charges having been generated in the semiconductor layer, (b) an accumulating capacitor for accumulating the electric charges having been collected by the collecting electrode, and (c) a TFT switch for reading out the electric charges having been accumulated by the accumulating capacitor.
In cases where a radiation image is to be recorded on the radiation image detector using the TFT as described above, firstly, a positive voltage is applied from a voltage source to the top electrode of the radiation image detector. Also, in this state, radiation carrying radiation image information of an object is irradiated from the side of the top electrode of the radiation image detector.
The radiation, which has thus been irradiated to the radiation image detector, passes through the top electrode and impinges upon the semiconductor layer. As a result, pairs of positive and negative charges are generated in the semiconductor layer by the irradiation of the radiation. Of the pairs of positive and negative charges having been generated in the semiconductor layer, the negative charges combine with the positive charges occurring in the top electrode and become extinct. Of the pairs of positive and negative charges having been generated in the semiconductor layer, the positive charges are collected as latent image charges by the collecting electrode of each of the pixels constituting the active matrix substrate. The positive charges having thus been collected are accumulated by the accumulating capacitor. The radiation image is recorded in this manner.
In cases where the radiation image having thus been recorded is to be read out from the radiation image detector, the TFT switch of the active matrix substrate is turned on by a control signal having been outputted from a gate driver, and the electric charges having been accumulated by the accumulating capacitor are read out. The electric charges having thus been read out are detected by a charge amplifier. In this manner, the image signal representing the radiation image is read out.
However, in the cases of the radiation image detector employed for the electrical read-out technique, at the time at which the positive voltage is applied to the top electrode in the manner described above, positive holes are injected from the top electrode into the semiconductor layer by the voltage application. As a result, as illustrated in FIG. 9, after the irradiation of the radiation has been ceased, the positive holes are injected from the top electrode. The thus injected positive holes are detected as a residual image current (as indicated by the hatching in FIG. 9). Therefore, noise is mixed in the image signal having been read out, and the image quality of the radiation image having been read out becomes bad.
Radiation image detectors utilizing amorphous selenium have also been proposed in, for example, the literatures of U.S. Pat. No. 6,642,534 and Japanese Unexamined Patent Publication No. 2001-177140. In the cases of the radiation image detectors described in these literatures, such that electric charges may be prevented from being injected from an electrode, to which a voltage is to be applied, into an amorphous selenium layer, it is proposed to locate a layer that is formed from Sb2S3 between the electrode and the amorphous selenium layer.
However, it has been found that, with the layer that is formed from Sb2S3, though the effects of blocking the injection of the electric charges are capable of being obtained, depending upon the layer thickness, the obtained effects are not always be sufficient, and the image quality of the radiation image having been read out is not always be capable of being kept good.